The present invention relates to a method for producing a magnetic recording medium, and particularly relates to a coating method for use in producing a magnetic recording medium using an extrusion-type coating head for coating the surface of a moving support with a coating composition continuously extruded onto the surface of the support having a uniform coating thickness.
Various coating methods have been employed for producing a magnetic recording medium, and of such methods a coating method using such an extrusion-type coating head as shown in FIG. 4 has often been used. In this coating method, coating is carried out while a coating head 10 is urged against a running support 1, while simultaneously a coating composition is extruded through a slit 4 between a doctor edge surface 2 and the support 1, thereby to coat the surface of the support 1 with the coating composition in a continuous manner, as illustrated in FIG. 4. Although the use of this method contributes to high-speed thin-film coating, the method has problems in that striped unevenness and widthwise deviations in film thickness are apt to be caused by impurities trapped on the coating surface in the small gap between the support 1 and the doctor edge surface 2. Various proposals have been made to overcome the foregoing problems, which can be classified as follows:
(1) Proposals in which the doctor edge surface 2 is formed with a desired curvature (R, R.sub.1 and R.sub.2) to provide a countermeasure against trapping of impurities. (See Japanese Patent Unexamined Publications Nos. Sho. 60-238179, Sho. 63-88080, and Hei. 2-265672.)
(2) Proposals in which the pressure (P.sub.w) of a coating composition produced by the lapping of the support 1 to the doctor edge surface 2 is defined so as to prevent striped unevenness and the like. (See Japanese Patent Unexamined Publication Nos. Sho. 62-117666 and Hei. 3-162).
(3) Proposals in which the diameter of a pocket 3 of a coating head, the clearance (d) and length (m) of the slit 4, the viscosity of a coating composition, etc., are defined so as to make the quantity of coating composition extruded onto the recording medium uniform in the widthwise direction of the slit. (See Japanese Patent Unexamined Publication No. Hei. 1-180266 and Japanese Patent Application No. Hei. 1-340394.)
However, in an extrusion-type coating apparatus with a doctor edge such as disclosed in Japanese Patent Unexamined Publications Nos. Sho. 60-238179, Sho. 63-88080, and Hei. 2-265672 mentioned in (1) above in which the doctor edge surface is curved, deviations of coating thickness due to widthwise unevenness of the support are apt to occur, although generation of striped unevenness caused by high-speed thin-film coating and impurity trapping may be prevented effectively. It is therefore difficult to perform coating stably and continuously.
In the approaches disclosed in Japanese Patent Unexamined Publication Nos. Sho. 62-117666 and Hei. 3-162 mentioned in (2) above, the coating composition pressure (P.sub.w) produced by the lapping of the support is controlled. In such methods, however, deviations in thickness of the coating film tend to occur in the case where the support is uneven in the widthwise direction.
In the methods disclosed in Japanese Patent Unexamined Publication No. Hei. 1-180266 and Japanese Patent Application No. Hei. 1-340394 mentioned in (3) above in which the size of the coating head is determined so as to make the quantity of the coating composition extruded from a slit top end portion uniform in the widthwise direction, deviations in thickness of the coating film are apt to occur depending on the shape of the support or the coating head. Accordingly, similar to the case of (1), there has been a problem that it is difficult to perform coating stably and continuously.
That is, in a coating method using an extrusion-type coating head it is indeed possible in the conventional technique (3) to obtain a product having no deviation in the thickness of the coated film if there is no widthwise unevenness (rigidity, thickness, tension, etc.) of the support 1. In an actual case, however, it is impossible as a practical matter to prepare a support having essentially no widthwise unevenness at all, and if it were possible to provide such a support, the support would be too expensive to use. It is therefore necessary to employ a support having some unevenness in the widthwise direction.